Known processes heretofore in use for producing a synthetic quartz glass for, e.g., an optical member such as a mirror or lens include a process which comprises forming a porous quartz glass base by a vapor-phase reaction method and heating this base to convert it into a transparent glass (see, for example, patent document 1).
In the process for synthetic-quartz-glass production disclosed in patent document 1, a porous quartz glass base of a substantially cylindrical shape is formed by the so-called VAD (vapor-phase axial deposition) method in which a silicon compound, e.g., silicon tetrachloride, is introduced into an oxyhydrogen flame to synthesize fine quartz glass particles through flame hydrolysis and the fine quartz glass particles are deposited on a rotating target. In the process, this base is heated to a temperature not lower than the vitrification temperature to convert the base into a transparent glass.
The synthetic quartz glass thus obtained contains hydroxyl groups (OH groups) typically in an amount of about 100 to 300 ppm because of the nature of the process. It is known that the distribution of concentration of these OH groups can be a factor which causes a refractive index distribution in the synthetic quartz glass. Various techniques for diminishing this refractive index distribution have been proposed (see, for example, patent document 2).
In the process for producing a synthetic quartz glass disclosed in patent document 2, an operation in which a synthetic quartz glass is heated to a temperature not lower than the softening point to cause the glass to undergo self-weight deformation is repeatedly conducted while changing the direction of the self-weight deformation for each operation to thereby diminish the refractive index distribution in the synthetic quartz glass.
Patent Document 1: JP-A-62-72536
Patent Document 2: JP-A-64-28240
However, it is difficult to completely eliminate the refractive index distribution in a synthetic quartz glass even by, e.g., the process for producing a synthetic quartz glass disclosed in patent document 2. A further measure against the refractive index distribution remaining in a synthetic quartz glass has been necessary.